The purpose of this work is to identify, isolate, and chemically define the peptide fragment of the chlamydial major outer membrane protein (MOMP) which contains the antigenic determinant that confers Chlamydia trachomati serotyping specificity. These antigenic determinants of the MOMP are exposed on the surface of viable chlamydiae and elicit neutralizing antibody in vitro; therefore they are attractive candidates for chlamydial vaccine development (see project Z01 AI 00216-03 LMSF). Purified chlamydial MOMP was digested with cyanogen bromide (CNBr) or succinylated and trypsin digested. The antigenic properties of the peptide fragments were analyzed by immunoblotting with monoclonal antibody directed against the serotype-specific epitope of the intact protein. A single 9K dalton CNBr fragment was identified that reacted with the sterotype specific monoclonal antibody. This peptide is currently being isolated by immunoadsorption using solid phase bound antibody. Amino acid sequencing of the 9K fragment and the parent MOMP have been performed. This information will be used to generate a synthetic oligonucleotide probe and to construct synthetic peptides. This should allow molecular cloning of the peptide gene and its synthetic biosynthesis which will provide the needed quantities fo the peptide to begin experimental in vivo animal model vaccine studies.